The present invention relates to a seismic source suitable for downhole use. More particularly, the present relates to a re-useable acoustic source which couples well with the formation surrounding a well bore.
In drilling a borehole to recover oil or other fluids from the earth, it is often helpful to turn or steer the downhole drill bit toward or away from subterranean targets. To facilitate this geophysical steering, drillers need to know drill bit location on the surface seismic section. The location of targets ahead of the bit is also required, as well as some warning or indication of drilling hazards such as over-pressured formations or thin, shallow gas intervals. Surface seismic surveys may be used to obtain this information, but resolution and depth location is poor because surface seismic surveys are time based (rather than depth based). For example, to determine the depth of a reflection, a speed of sound for the formation must be known. Consequently, these systems require depth calibration to accurately determine locations of target horizons or drilling hazards. Traditionally, this calibration has been provided by either offset well sonic data or wireline checkshot data in the current well. Offset data is often inadequate due to horizontal variations in stratigraphy between wells. Wireline checkshots require tripping (i.e., removing) the bit out of the hole and are often prohibitively expensive for this reason.
During surface seismic surveys, a plurality of seismic sources and seismic receivers are placed on the surface of the earth. The seismic sources are separately triggered to generate seismic waves. These seismic waves travel downward through the earth until reflected off some underground object or change in rock formation. The reflected seismic waves then travel upward and are detected at the seismic receivers on the surface. One or more clocks at the surface measure the time from generation of the seismic waves at each source to the reception of the seismic waves at each receiver. This gives an indication of the depth of the detected object underground. However, the exact speed of sound for these seismic waves is unknown, and thus, the exact depth of the detected object is also unknown. To more closely measure the exact speed of sound, a xe2x80x9cwireline checkshotxe2x80x9d may be used to calibrate depth measurements. During a xe2x80x9cwireline checkshot,xe2x80x9d a receiver on a xe2x80x9cwirelinexe2x80x9d is lowered a known distance into an already-drilled borehole. A surface seismic source is then triggered and the time is measured for the seismic wave to travel to the wireline receiver. Because the depth of the wireline receiver is known, an average interval velocity indicating the average speed of the seismic wave can be determined with some degree of accuracy.
A more direct solution to the depth resolution problem is known as xe2x80x9cvertical seismic profilingxe2x80x9d, or VSP. In VSP, an array of receivers is located along the vertical length of a borehole and a plurality of seismic sources are located on the surface. As before, the seismic sources produce seismic waves that propagate through the earth and reflect from interfaces in the formation. The receivers detect both the reflections and an initial, un-reflected seismic wave that has propagated through the formations (unlike the surface surveys where the initial seismic wave only propagates along the surface). The initial seismic wave provides additional information to allow accurate determination of formation interface depths.
An alternative, but related, solution is known as xe2x80x9creverse vertical seismic profilingxe2x80x9d. This approach transposes the downhole location of the receivers with the surface location of the seismic sources. Some drill bits generate a significant amount of seismic noise that can be detected by receivers on the surface. Since the position of the drill bit varies as the hole is drilled, enough information can be gathered to build model of the formation. While reverse VSP is not limited to using drill bits as seismic sources, the prior art efforts have been focused in this direction. However, there are many important situations in which a drill bit is inadequate to the task of generating the requisite seismic energy. For example, diamond bits offer numerous advantages to drilling, but they cut too quietly. Soft formations, when cut by any drill bit, also fail to generate sufficient noise.
Some attempts have been made to create alternative downhole seismic sources. A source by Klaveness, (U.S. Pat. No. 5,438,170, hereby incorporated herein by reference), is akin to a drilling jar that causes sudden, forceful ejection of fluid into the wellbore. However, this source generates large tool modes, large tube waves, and poor seismic signals. Another known source vibrates the bit with a piezoelectric transducer. It has a very short range and can only be used with downhole receivers. Unfortunately, when the receivers are downhole with the source, high-pressure formations ahead of the bit cannot be distinguished from other reflective boundaries. An air gun has also been placed downhole, but requires air lines from the surface. Air guns and water guns are not ideal downhole sources because they are localized sources that create large tube waves, require intrusive pressure systems, and may damage the formation. As an experimental downhole source, a explosive charge has been used successfully to reduce formation damage and tube wave intensity. However, explosive charges are not re-useable and may be dangerous in some drilling environments.
The above problems are solved by an axially extended downhole seismic source. In one embodiment, the seismic source includes multiple pressure storage chambers, each having an inlet valve and an outlet valve. The inlet valve is coupled between the pressure storage chamber and the interior of the drill string, and the outlet valve is similarly coupled between the pressure storage chamber and the annular space around the drill string. A compressible fluid may be provided in the pressure storage chambers and pistons may be positioned to contact compressible fluid. For each pressure storage chamber, an inlet piston contacts the compressible fluid and fluid inside the drill string, while an outlet piston contacts the compressible fluid and fluid in the annular space around the drill string. When the outlet valve is closed, the inlet valve can be opened to allow pressure inside the drill string to compress the compressible fluid inside the pressure storage chamber. Subsequently closing the inlet valve and opening the outlet valve causes fluid to be ejected into the annular space, thereby generating seismic waves. The use of multiple pressure storage chambers allows the pressure front from the seismic source to be extended axially to advantageously increase the fraction of seismic energy transmitted into the formation while preventing damage to the formation.